Implement User Qualifications

[Zomtir]

Should be able to handle the DOSB license for "Übungsleiter", "Trainer" and "Vereinsmanager".

Draft

pub struct Qualification {
    pub qualification_id: u8,
    pub title: String,
    pub url: Option<String>,
    pub notes: Option<String>,
    pub valid_date: Option<chrono::NaiveDate>,
}

[Zomtir]

Implemented in 06e08b4 except the URL/path do the document/scan/pdf/image. The note field is probably not needed.

[Zomtir]

Implemented a file_url in 46d2137. A download API for the PDF itself is yet to be done.